Cellular (e.g. GSM or UMTS (3G)) architectures include a core network (CN) and a radio access network (RAN). Mobile terminals, once authenticated to a given network, place calls by first reserving a suitable channel on the RAN and then requesting connection to other terminals giving a called number (MSISDN) to identify the called party. The CN looks up at least one database to locate the called party terminal and to allow a call to be terminated at that called party terminal. Where the called party terminal belongs to a second network (different from the given network with which the calling party terminal is authenticated), this requires the establishment of a radio access channel from a mobile terminal to the core network of a first cellular network; the establishment of a link between the first network and a second cellular network; and the establishment of a further radio access channel to a second mobile terminal via the second cellular network.
The Internet protocol multimedia subsystem (IMS) is a set of core network servers logically sited behind the gateway GPRS support node (GGSN) in the packet switched domain of a cellular telecommunications network. The servers process signalling between terminals. The aim of IMS is to allow users such as mobile telephone network service providers to provide services to their subscribers as efficiently and effectively as possible and independently of the manner by which its network is accessed. The IMS architecture is intended to support, for example, the following communication types: voice, video, instant messaging, “presence” (a user's availability for contact), location based services, e-mail and web. Further communication types are likely to be added in the future.
3GPP has chosen session initiation protocol (SIP) for managing these sessions. The SIP protocol is a session-based protocol designed to establish IP-based communication sessions between two or more terminals. SIP is used as a means of signalling, end-to-end, the initiation, modification and termination of packet switched sessions. Once a SIP session has been established, communication between the terminals can be carried out using a variety of different protocols (for example, those designed for streaming audio and video). These protocols are defined in SIP session initiation messages.
With IMS, users are no longer restricted to a separate voice call or data session. Sessions can be established between terminals that allow a variety of communication types to be used and media to be exchanged. The sessions are dynamic in nature in that they can be adapted to meet the needs of the end users. For example, two such users might start a session with an exchange of instant messages and then decide that they wish to change to a voice call, possibly with video. This is all possible within the IMS framework. If a user wishes to send a file to another user and the users already have a session established between each other (for example, a voice session), the session can be re-defined to allow a data file exchange to take place. This session redefinition is transparent to the end user.
International calls made in conventional fixed line (e.g. PSTN) or cellular (e.g. GSM or UMTS (3G)) telecommunications architectures are readily identifiable as such to a calling party (originator) because making such a call requires the use of an international dialing code. In addition, network service providers are conventionally allocated dialing codes, which codes each network service provider has been allocated being publicly available. Accordingly, it has been possible for a calling party to determine that a conventional call will be an “international” call before making the call.
The SIP protocol identifies a terminal by an address of the form “user@domain”. If a user (the calling party) wishes to establish a session, which may be for any supported communication type, with another user (the called party), signalling will initially be directed to the called party using an address of this form. Such an address provides no indication of the geographical location of the called party. Even if, for example, the domain includes an identifier of a country, such as the domain “.co.uk”, such a domain can be the address of a terminal neither located in that country nor registered with a network service provider of that country. As a result, the calling party, using SIP, does not know in advance of initiating a session whether the called party's terminal is registered in the same country as the calling party's terminal, or in another country. The charge payable by the calling party to the network service provider with which the calling party is registered for the session is typically dependent upon the country in which the called party's terminal is registered. An international call conventionally attracts a higher charge rate.
Furthermore each party that wishes to access the additional services offered by the IMS needs to subscribe to the required services.
Clearly, the calling party may wish to know the applicable charge rate to confirm that charges payable to the network service provider will not exceed what should be expected.
One disadvantage of conventional international call connection is the requirement for the reservation of a significant amount of network resource in both originating and terminating networks—the call establishment signalling inevitably requires HLR resource as well as MSC/VLR resource—as well as the provision of a suitable international carrier interface between the two networks. In turn, the use of additional resource is reflected in the cost of the service provided to customers.
IP call connection reduces the signalling overhead but imposes other constraints—notably the need for IP multimedia Subsystem (IMS) subscriptions for both originator and terminating party.